curiocity about impedance

ok, question...... now I know impedance, and sensitivity play into how easy a can is to drive. something puzzles me. I have the AKG k-160 600x2 ohm 98 dec sensitivity headphones, and a myriad of lower impedance, higher sensitivity headphones. so the question is, how, and why can the 600 ohm k-160 be driven nicely loud directly from my iphone, and other small sources, while say the k-701 (68 ohm, and 105 sens), or the he-400 (48 ohm, 98 sens) require a good ooompf from both the iphone, and the e11? I was really expecting to struggle to drive the k160. Now granted heh the k160 doesnt sound like gold......far from it, tho very good for being so old